# 可用在命令中的前缀
# @ 不输出命令
# - 忽略命令中错误
# + 执行命令
 
 test:
	@if [ ! -d xxx ]; \
	then             \
	  mkdir xxx;     \
	fi
	@echo hahaha

file :=   vs     2017
space = $(empty) $(empty)
space-to-question = $(subst $(space),?,$1)
file-test:
	@echo $(call space-to-question,$(file))
